Andreja Susnik

Dependency of geodynamic parameters on the GNSS constellation

Significant differences in time series of geodynamic parameters determined with different Global Navigation Satellite Systems (GNSS) exist and are only partially explained. We study whether the different number of orbital planes within a particular GNSS contributes to the observed differences by analyzing time series of geocenter coordinates (GCCs) and pole coordinates estimated from several real and virtual GNSS constellations: GPS, GLONASS, a combined GPS/GLONASS constellation, and two virtual GPS sub-systems, which are obtained by splitting up the original GPS constellation into two groups of three orbital planes each. The computed constellation-specific GCCs and pole coordinates are analyzed for systematic differences, and their spectral behavior and formal errors are inspected. We show that the number of orbital planes barely influences the geocenter estimates. GLONASS’ larger inclination and formal errors of the orbits seem to be the main reason for the initially observed differences. A smaller number of orbital planes may lead, however, to degradations in the estimates of the pole coordinates. A clear signal at three cycles per year is visible in the spectra of the differences between our estimates of the pole coordinates and the corresponding IERS 08 C04 values. Combinations of two 3-plane systems, even with similar ascending nodes, reduce this signal. The understanding of the relation between the satellite constellations and the resulting geodynamic parameters is important, because the GNSS currently under development, such as the European Galileo and the medium Earth orbit constellation of the Chinese BeiDou system, also consist of only three orbital planes.

Impact of GNSS orbit modeling on LEO orbit and gravity field determination

On January 4, 2015 the Center for Orbit Determination in Europe (CODE) changed the solar radiation pressure modeling for GNSS satellites to an updated version of the empirical CODE orbit model (ECOM). Furthermore, since September 2012 CODE operationally computes satellite clock corrections not only for the 3-day long-arc solutions, but also for the non-overlapping 1-day GNSS orbits. This provides different sets of GNSS products for Precise Point Positioning, as employed, e.g., in the GNSS-based precise orbit determination of low Earth orbiters (LEOs) and the subsequent Earth gravity field recovery from kinematic LEO orbits. While the impact of the mentioned changes in orbit modeling and solution strategy on the GNSS orbits and geophysical parameters was studied in detail, their implications on the LEO orbits were not yet analyzed. We discuss the impact of the update of the ECOM and the influence of 1-day and 3-day GNSS orbit solutions on zero-difference LEO orbit and gravity field determination, where the GNSS orbits and clock corrections, as well as the Earth rotation parameters are introduced as fixed external products. Several years of kinematic and reduced-dynamic orbits for the two GRACE LEOs are computed with GNSS products based on both the old and the updated ECOM, as well as with 1- and 3-day GNSS products. The GRACE orbits are compared by means of standard validation measures. Furthermore, monthly and long-term GPS-only and combined GPS/K-band gravity field solutions are derived from the different sets of kinematic LEO orbits. GPS-only fields are validated by comparison to combined GPS/K-band solutions, while the combined solutions are validated by analysis of the formal errors, as well as by comparing them to the combined GRACE solutions of the European Gravity Service for Improved Emergency Management (EGSIEM) project.

Reprocessing campaign in the framework of the EGSIEM project at AIUB

In the framework of the European Gravity Service for Improved Emergency Management (EGSIEM) project, monthly gravity field solutions derived from the Gravity Recovery and Climate Experiment (GRACE) mission will be combined. Since an improved reference frame is a prerequisite for precise orbit and gravity field determination, a reprocessing campaign (Repro15) was initiated at the Astronomical Institute of the University of Bern (AIUB), with more than 250 globally distributed tracking stations of the International GNSS Service (IGS), homogeneously processed for the interval between 2003 to the end of 2014. Since the Low Earth Orbiting satellites (LEOs) are tracking with a higher sampling than the usual 30 s of the IGS tracking stations, we included the 1 Hz dataset provided by the IGS real-time pilot project/service. The procedures established by the Center for Orbit Determination in Europe (CODE, hosted at AIUB), have been applied for the reprocessing. The new Empirical CODE Orbit Model (extended ECOM) has been applied to model the GNSS orbits. The procedure to densify the satellite clock corrections down to 5 s is applied to the GLONASS satellites for the first time.

Evaluation of ITRF2014 Solutions

For the most recent International Terrestrial Reference Frame (ITRF) realization three candidates have been provided, namely an ITRF2014 solution by IGN, DTRF2014 by DGFI-TUM, and JTRF2014 by JPL. There are significant differences in the way how these solutions have been generated, which parametrization has been applied, and how the solutions from the different space-geodetic techniques are combined.

CODE ultra-rapid product series for the IGS

CODE, the Center for Orbit Determination in Europe, is a joint venture of the following four institutions: Astronomical Institute, University of Bern (AIUB), Bern, Switzerland; Federal Office of Topography swisstopo, Wabern, Switzerland; Federal Agency of Cartography and Geodesy (BKG), Frankfurt a. M., Germany; Institut fur Astronomische und Physikalische Geodasie, Technische Universitat Munchen (IAPG, TUM), Munich, Germany. It acts as a global analysis center of the International GNSS Service (IGS). The operational computations are performed at AIUB using the latest development version of the Bernese GNSS Software. In this context an ultra-rapid solution series is generated considering GPS and GLONASS satellites. It is updated several times per day and contains 24 hours of observed and 24 hours of predicted orbit interval. More details are available in: Lutz, S., G. Beutler, S. Schaer, R. Dach, A. Jaggi; 2014: CODEs new ultra-rapid orbit and ERP products for the IGS. GPS Solutions. DOI 10.1007/s10291-014-0432-2

CODE Analysis center: Technical Report 2015

Applications of the Global Navigation Satellite Systems (GNSS) to Earth Sciences are numerous. The International GNSS Service (IGS), a voluntary federation of government agencies, universities and research institutions, combines GNSS resources and expertise to provide the highest–quality GNSS data, products, and services in order to support high–precision applications for GNSS–related research and engineering activities. This IGS Technical Report 2015 includes contributions from the IGS Governing Board, the Central Bureau, Analysis Centers, Data Centers, station and network operators, working groups, pilot projects, and others highlighting status and important activities, changes and results that took place and were achieved during 2015.